The present invention pertains in general to the fabrication of semiconductor devices and, in particular, to selfaligned methods for manufacturing high density integrated circuits.
A recent advance in the manufacture of high density integrated circuits has been the development of epitaxial base devices. Devices of this type are formed within thin epitaxial layers grown on the surface of a semiconductor substrate. One prior art method uses oxide to form the isolation region around a device and a buried layer under the device to form a collector region. Another method is collector diffused isolation (CDI) where an annular doped region is diffused down to contact a buried layer thus simultaneously delineating the base-collector and the collector-isolation junctions of the device. The major advantage in epitaxial base devices is that much smaller devices can be fabricated without causing emitter-collector shorts. Studies show that the incidence of emitter-collector shorts is inversely proportional to the width of the base region of a integrated circuit device. Thin epitaxial layers which can be formed with a uniform low concentration level allow the fabrication of devices with a wider base region with no sacrifice in current gain.
Another recent advance in fabricating high density integrated circuits is the use of self-aligning techniques. These techniques use selectively etchable thin layers with openings in one layer forming the etching mask for openings in subsequent layers to form a given device region. In oxide structures, self-aligning techniques have been used to simplify and compact the formation of base contacts and emitter regions within a given device. Oxide isolation precludes, however, the self-alignment of these regions with respect to the isolation region since the openings in the masking layer cannot be preserved during the long oxide growth cycle required to form the isolation region.
The diffusions used to form the collector region in a CDI integrated circit do not in themselves preclude selfalignment techniques but there is another reason why total self-alignment of emitter region, base metal contact and collector region cannot be obtained in conventional CDI structures. This is the fact that the overall surface of the base region of an epitaxial base device must receive higher doping if emitter efficiencies are to be maintained. Highest emitter efficiencies occur when injection of carriers from the emitter can be maximized in the base region directly under the emitter and minimized in the sidewall regions of the periphery of the emitter. Injection is a function of the impurity concentration (doping) of the emitter with respect to the base. Thus to maximize injection under the emitter, doping must be low in this region of the base whereas to minimize injection in emitter sidewall, doping must be high in this region of the base.
These requirements for improved emitter efficiency can be met by forming a thin, more highly doped layer at the surface of the base region. The depth of this more highly doped layer must be less than the depth of the emitter regions so that injection is retarded only in the sidewall regions of the emitter. A thin layer of this type can be formed by conventional diffusion techniques but the capability for self-alignment is then lost because the selectively etchable masking layers must be removed from the surface in order to accomplish the diffusion. Thus, prior art methods for the fabrication of epitaxial base integrated circuit structures have shown methods for self-alignment of regions interior to a given device or the formation of unpatterned shallow doped regions over the surface of the given device but not both.